CVE-2024-35932 in Linux
Summary
by MITRE • 05/19/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/vc4: don't check if plane->state->fb == state->fb
Currently, when using non-blocking commits, we can see the following kernel warning:
[ 110.908514] ------------[ cut here ]------------
[ 110.908529] refcount_t: underflow; use-after-free.
[ 110.908620] WARNING: CPU: 0 PID: 1866 at lib/refcount.c:87 refcount_dec_not_one+0xb8/0xc0
[ 110.908664] Modules linked in: rfcomm snd_seq_dummy snd_hrtimer snd_seq snd_seq_device cmac algif_hash aes_arm64 aes_generic algif_skcipher af_alg bnep hid_logitech_hidpp vc4 brcmfmac hci_uart btbcm brcmutil bluetooth snd_soc_hdmi_codec cfg80211 cec drm_display_helper drm_dma_helper drm_kms_helper snd_soc_core snd_compress snd_pcm_dmaengine fb_sys_fops sysimgblt syscopyarea sysfillrect raspberrypi_hwmon ecdh_generic ecc rfkill libaes i2c_bcm2835 binfmt_misc joydev snd_bcm2835(C) bcm2835_codec(C) bcm2835_isp(C) v4l2_mem2mem videobuf2_dma_contig snd_pcm bcm2835_v4l2(C) raspberrypi_gpiomem bcm2835_mmal_vchiq(C) videobuf2_v4l2 snd_timer videobuf2_vmalloc videobuf2_memops videobuf2_common snd videodev vc_sm_cma(C) mc hid_logitech_dj uio_pdrv_genirq uio i2c_dev drm fuse dm_mod drm_panel_orientation_quirks backlight ip_tables x_tables ipv6
[ 110.909086] CPU: 0 PID: 1866 Comm: kodi.bin Tainted: G C 6.1.66-v8+ #32
[ 110.909104] Hardware name: Raspberry Pi 3 Model B Rev 1.2 (DT)
[ 110.909114] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 110.909132] pc : refcount_dec_not_one+0xb8/0xc0
[ 110.909152] lr : refcount_dec_not_one+0xb4/0xc0
[ 110.909170] sp : ffffffc00913b9c0
[ 110.909177] x29: ffffffc00913b9c0 x28: 000000556969bbb0 x27: 000000556990df60
[ 110.909205] x26: 0000000000000002 x25: 0000000000000004 x24: ffffff8004448480
[ 110.909230] x23: ffffff800570b500 x22: ffffff802e03a7bc x21: ffffffecfca68c78
[ 110.909257] x20: ffffff8002b42000 x19: ffffff802e03a600 x18: 0000000000000000
[ 110.909283] x17: 0000000000000011 x16: ffffffffffffffff x15: 0000000000000004
[ 110.909308] x14: 0000000000000fff x13: ffffffed577e47e0 x12: 0000000000000003
[ 110.909333] x11: 0000000000000000 x10: 0000000000000027 x9 : c912d0d083728c00
[ 110.909359] x8 : c912d0d083728c00 x7 : 65646e75203a745f x6 : 746e756f63666572
[ 110.909384] x5 : ffffffed579f62ee x4 : ffffffed579eb01e x3 : 0000000000000000
[ 110.909409] x2 : 0000000000000000 x1 : ffffffc00913b750 x0 : 0000000000000001
[ 110.909434] Call trace:
[ 110.909441] refcount_dec_not_one+0xb8/0xc0
[ 110.909461] vc4_bo_dec_usecnt+0x4c/0x1b0 [vc4]
[ 110.909903] vc4_cleanup_fb+0x44/0x50 [vc4]
[ 110.910315] drm_atomic_helper_cleanup_planes+0x88/0xa4 [drm_kms_helper]
[ 110.910669] vc4_atomic_commit_tail+0x390/0x9dc [vc4]
[ 110.911079] commit_tail+0xb0/0x164 [drm_kms_helper]
[ 110.911397] drm_atomic_helper_commit+0x1d0/0x1f0 [drm_kms_helper]
[ 110.911716] drm_atomic_commit+0xb0/0xdc [drm]
[ 110.912569] drm_mode_atomic_ioctl+0x348/0x4b8 [drm]
[ 110.913330] drm_ioctl_kernel+0xec/0x15c [drm]
[ 110.914091] drm_ioctl+0x24c/0x3b0 [drm]
[ 110.914850] __arm64_sys_ioctl+0x9c/0xd4
[ 110.914873] invoke_syscall+0x4c/0x114
[ 110.914897] el0_svc_common+0xd0/0x118
[ 110.914917] do_el0_svc+0x38/0xd0
[ 110.914936] el0_svc+0x30/0x8c
[ 110.914958] el0t_64_sync_handler+0x84/0xf0
[ 110.914979] el0t_64_sync+0x18c/0x190
[ 110.914996] ---[ end trace 0000000000000000 ]---
This happens because, although `prepare_fb` and `cleanup_fb` are perfectly balanced, we cannot guarantee consistency in the check plane->state->fb == state->fb. This means that sometimes we can increase the refcount in `prepare_fb` and don't decrease it in `cleanup_fb`. The opposite can also be true.
In fact, the struct drm_plane .state shouldn't be accessed directly but instead, the `drm_atomic_get_new_plane_state()` helper function should be used. So, we could stick to this check, but using `drm_atomic_get_new_plane_state()`. But actually, this check is not re ---truncated---
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Analysis
by VulDB Data Team • 09/23/2025
The vulnerability CVE-2024-35932 resides within the Linux kernel's graphics subsystem, specifically in the vc4 driver responsible for handling graphics operations on Raspberry Pi devices. This flaw manifests as a use-after-free condition triggered during non-blocking display commits, leading to potential system instability and memory corruption. The root cause stems from improper reference counting mechanisms when managing framebuffer objects during display plane updates, where direct access to plane state structures bypasses proper atomic state handling protocols.
The technical flaw occurs when the kernel attempts to decrement reference counts on graphics buffer objects without maintaining proper synchronization between prepare and cleanup operations. The warning message indicates a refcount_t underflow, which typically occurs when a reference counter is decremented below zero, suggesting that cleanup functions are either called more times than prepare functions or that the reference counting logic itself is flawed. The call trace reveals this issue originates in vc4_bo_dec_usecnt, which is invoked through vc4_cleanup_fb during atomic commit processing, ultimately stemming from drm_atomic_helper_cleanup_planes.
This vulnerability presents significant operational impact for systems relying on the vc4 driver, particularly those running media applications like Kodi or other graphics-intensive software on Raspberry Pi hardware. The use-after-free condition can lead to arbitrary code execution, system crashes, or denial of service attacks, making it a critical concern for embedded systems and media centers. The vulnerability specifically affects non-blocking commit operations, which are common in modern graphics applications that require smooth, uninterrupted display updates.
Mitigation strategies involve implementing proper atomic state handling through the drm_atomic_get_new_plane_state() helper function instead of direct struct drm_plane.state access. This ensures consistent reference counting across all plane state transitions and prevents the imbalance between prepare and cleanup operations. System administrators should update to patched kernel versions that address this specific reference counting issue. Additionally, monitoring for kernel warnings related to refcount_t underflows and implementing proper error handling in graphics applications can help detect and prevent exploitation of this vulnerability. This issue aligns with CWE-415: Double Free and CWE-476: NULL Pointer Dereference, and maps to ATT&CK techniques involving privilege escalation and system compromise through kernel vulnerabilities.
The fix requires modification of the vc4 driver's atomic commit handling to properly utilize atomic state helpers, ensuring that reference counting operations maintain consistency regardless of commit type. This change should be validated across various display scenarios to ensure no regressions in functionality while resolving the underlying use-after-free condition.